Hydraulic valve clearance adjusting elements serve to adjust the clearance which, in transmitting the cam lift of a camshaft to a gas exchange valve of the internal combustion engine, is formed between the transmission elements due to wear or thermal expansion. The intention in using the adjusting element is to achieve a quiet and wear-resistant operation of the valve train and the greatest possible conformity between the cam lobe and the lift of the gas exchange valve.
Such adjusting elements in each case have a control valve embodied as a non-return valve, which has a closing member, such as a ball, and a control valve spring, which acts upon the closing member. In the standard type of control valve the control valve spring acts upon the closing member in the closing direction. This largely closes the control valve so that there is no idle travel of the valve clearance adjusting element. In this embodiment there is a risk of pumping up the adjusting element, producing a “negative valve clearance”. DE 198 44 202 A1 demonstrates a valve timing gear of the aforementioned type, in which the valve clearance adjusting element is embodied as an adjustable circular-cylindrical cam follower. As can be seen from the drawing, the control valve spring acts upon the closing member in the closing direction, so that the control valve is largely closed.
The disadvantages of known adjusting elements are avoided by control valves, the control valve spring of which acts upon the closing member in the opening direction, or in which a spring is entirely dispensed with. Adjusting elements having a control valve of this type are referred to as reverse-spring elements, owing to the inverse arrangement of the control valve spring, or in the absence of a spring as freeball elements.
These exert a positive influence on the thermodynamics, the pollutant emissions and the mechanical stressing of the internal combustion engine and are therefore being increasingly used.
Whereas in the standard design type the control valve is closed in the base circle area of the cam owing to the spring force of the control valve spring, with a reverse-spring element the control valve in this area is kept open by the force of the control valve spring, or in the case of a freeball element is not forcibly closed. Since such an element can only be closed by hydrodynamic and hydrostatic forces due to the flow of lubricating oil commencing at the beginning of the cam lobe and flowing from the high-pressure chamber to the low-pressure chamber, the element always has an idle travel before the valve lift of the gas exchange valve commences. The extent of the idle travel at any engine speed depends on the length of the control valve closing time and this in turn depends on the viscosity/density of the lubricating oil which, as is known, is used as hydraulic fluid.
To close the control valve of a reverse-spring/freeball element a so-called critical lubricating oil velocity is required. This varies as function of the lubricating oil viscosity and hence of the lubricating oil temperature. At high lubricating oil viscosity/density, that is to say at low lubricating oil temperatures, the critical lubricating oil velocity is lower and is therefore attained more rapidly than at low lubricating oil viscosity, that is to say high lubricating oil temperatures. In cold starting this leads to a shorter closing time of the control valve and hence to a smaller idle travel than in the engine at operating temperature. A small idle travel means a large valve overlap, however. This results in a large internal exhaust gas recirculation, which causes an uneven, low idling speed. Although this can be improved by increasing the idling speed, this is achieved at the expense of the pollutant emissions and the fuel consumption.
With reverse-spring/freeball elements the closing member of the control valve is therefore open in the base circle of the cam. To close the control valve, a volume flow must flow past the closing member, which causes a pressure differential on the closing member, with the result that the latter closes the control valve. Reverse-spring/freeball elements are disclosed, for example, by EP 1 298 287 A2, JP 61-185607 and U.S. Pat. No. 4,054,109. These demonstrate adjusting elements in each of which the control valve has a ball as closing member.
In conventional hydraulic valve clearance adjusting elements the control valve spring of the control valve embodied as a non-return valve is therefore arranged below the valve ball or the closing member and the piston forming the valve seat. At the start of the base circle phase of the camshaft cam, the control valve opens in order to replenish from the low-pressure chamber of the element that quantity of oil used as hydraulic medium, previously forced out of the high-pressure chamber of the element during the lifting phase, and in order to adjust an existing valve clearance through a corresponding fresh intake of oil into the high-pressure chamber of the element. Otherwise the control valve is closed during the base circle phase of the cam. The working of a conventional adjusting element imposes certain requirements on the design of the cam contacts.
Element-shortening base circle eccentricities in the conventional adjusting element can cause the engine or gas exchange valve to accidentally open in the base circle phase of the cam, which leads to corresponding mechanical and thermodynamic disadvantages.
Element-lengthening base circle eccentricities in the conventional adjusting element can cause the valve ball to open the non-return valve, so that the element draws oil from the low-pressure chamber into the high-pressure chamber. If this happens before the engine or gas exchange valve starts to lift, the control valve, opened owing to the intake, has first to close again before the cam lift can be transmitted to the gas exchange valve. The retarded function associated therewith produces an undesirable valve lift loss, which likewise leads to corresponding mechanical and thermodynamic disadvantages.